In disc drives, digital data are written to and read from a thin layer of magnetizable material on a surface of one or more rotating discs. Write and read operations are performed through a transducer which is carried in a slider body. The slider and transducer are sometimes collectively referred to as a head, and typically a single head is associated with each disc surface. When the transducer is a magnetoresistive (MR) type sensor, the combination of the slider and the transducer are frequently referred to as an MR head. The heads are selectively moved under the control of electronic circuitry to any one of a plurality of circular, concentric data tracks on the disc surface by an actuator device.
The transducer of an MR head provides a read back signal, indicative of the data read from the disc surface, to a preamplifier circuit. After amplification by the preamplifier, the read back signal is provided to a channel integrated circuit (IC) of the disc drive. The channel IC detects transition levels in the read back signal and provides an output signal which is indicative of the data states of the individual bits in the read back signal data stream, and thereby of data stored on the disc surface. An example of a channel IC is the IC device sold by Silicon Systems Incorporated under the product number SSI 4911C.
A phenomena associated with MR heads is baseline shift in the read back signal. Baseline shift is a low frequency problem which cannot be corrected by channel ICs such as the SSI 4911C. Baseline shift is the shift of the steady state value of the read back signal to values above and/or below the intended baseline, which is typically 0 volts. Baseline shift renders it difficult for the channel IC to detect levels in the read back signal. Attempts to compensate for baseline shift using software algorithms have proven to be difficult and costly to implement. Further, compensating for baseline shift in the read back signal can be a complex task since the read back signal can have a wide range of frequencies (i.e., can represent a wide range of data rates).
The present invention provides a solution to this and other problems, and offers other advantages over the prior art.